As a Library Trustee, I take a more than casual interest in what goes on with our extended library network. I know that there are already copies of Harry Potter and the Half-Blood Prince being processed for our system, which covers forty-one town and college libraries. If you click here, you get the Top 50 requests for books in the network, and it's no surprise that Harry Potter is number one.
But for more specific information... If you click here, you should get the network's page for the Harry Potter book. And near the top of the page, you will find the number of requests that have been made for the book:
"1351 holds on First Copy Returned"
I imagine the number might climb throughout the course of the day...